An hour before serving put a martini glass in the freezer.
When ready to serve, fill a cocktail shaker or small pitcher with ice. Add the vermouth and then the gin. Cover and shake vigorously, or stir, until combined and chilled, about 30 seconds. (In general, by the time the shaker mists up the drink is ready.) Strain the martini into the chilled glass. Add the olive and serve.